Difference between Group By and Partition By.

Group By Clause	Partition By Clause
Reduces the no. of records	No. of records will not be reduced
Any non group by column is not allowed in the select clause. Example : SELECT deptno ,COUNT(*) DEPT_COUNT FROM emp GROUP BY deptno;	Any non group by column is  allowed in the select clause. Example: SELECT empno, deptno ,COUNT(*) OVER (PARTITION BY deptno) DEPT_COUNT FROM emp;
Group by actually groups the result set returning one row per group.	Analytic functions (Partition by) give aggregate result they do not group the result set means They return the group value multiple times with each record.

How to find duplicate records if one of the column has unique value.

SELECT Count(*)
FROM   (
        SELECT DISTINCT
               Col1
             , Col2
             , Col3
        FROM   table_name
       ) As AAA

Difference Between IN and EXISTS

	IN	EXIST
1	IN is faster than EXISTS, when the sub-query results is very small.	EXISTS is much faster than IN, when the sub-query results is very large.
2	IN can't compare anything with null, so it has no result for null. For Example:Select * from Table where Table_ID in (select null) Above one would return empty.	EXISTS can compare everything with null. Select *  from Table where exists (select null). Above query will return all the records
3	If you are using the IN operator, the SQL engine will scan all records fetched from the inner query	On the other hand if we are using EXISTS, the SQL engine will stop the scanning process as soon as it found a match.
4	If Most of the filter criteria is in subquery then better to use IN	If most of the filter criteria is in main query then better to use EXISTS.

Difference Between VIEW and Materialized View

View	Materialized view
1.  In Views query result is not stored in the disk or database.	1. Materialized view allows to store query result in disk or table.
2. When we create view using any table, rowid of view is same as original table.	2. In case of Materialized view rowid is different.
3.  In case of View we always get latest data.	3. Materialized view we need to refresh the view for getting latest data.
4. In case of view we don't need extra trigger or some automatic method to refresh the view.	4. In case of Materialized view we need extra trigger or some automatic method so that we can keep Materialized view refreshed.

Difference Between CASE and DECODE

CASE	DECODE
1.CASE expression complies with ANSI SQL	1. DECODE is specific to Database vendors (like Oracle, SQL Server)
2. Case can compare discrete as well as Range value	2. DECODE compare discrete values not Range
3. CASE can work with logical operators, predicates and searchable subqueries.	3. DECODE can work with only scaler values
4. CASE is a statement.	4. DECODE is a function.
5. CASE expects datatype consistency.	5. DECODE does not expects datatype consistency.

Cumulative Sum or LAG function Example

 SELECT EMPNO, ENAME, SAL, NVL(SAL,0)+LAG(SAL,1,0) OVER (ORDER BY SAL)  SUM_SAL
 FROM EMP
 ORDER BY SAL

EMPNO	ENAME	SAL	SUM_SAL
7369	SMITH	800	800
7900	JAMES	950	1750
7876	ADAMS	1100	2050
7521	MORGAN	1250	2350
7654	MARTIN	1250	2500
7934	MILLER	1300	2550
7844	TURNER	1500	2800
7499	SMALL	1600	3100
7782	FOOTE	2450	4050
7698	BLAKE	2850	5300
7566	NORGAARD	2975	5825
7788	SCOTT	3000	5975
7902	LOFSTROM	3000	6000
7839	ELLISON	5000	8000

Popular Personal interview Question

Question: What is your weakness?

Ans: I concentrate on one thing at a time.

Question: Reason behind leaving your last job?

Ans: Looking for better opportunities and enhancing my skill set.

Question: what are your strengths?

Ans: I am a quick learner and a great team player.

Question: why should we hire you?

Ans: If you hire me it will be a great platform to show  my skills. Whatever goals I set ensure to complete them within stipulated time.

Question: Do you consider yourself successful?

Ans: Yes. Apart from appraisals I think I have earned a bunch of good colleagues in my life.

Question: Why have you been unemployed for such a long time?

Ans: I enrolled myself for advanced personality development course along with some freelance work.

Question: How long would you expect to work for us if hired?

Ans: As long as  I feel challenged professionally.

Question: Describe your management style.

Ans: I will be constantly keeping tab of assigned work with my subordinates and seniors, finishing the assigned job before deadlines.

Question: Are you a team player?

Ans: Yes, the team which I was a part of have successfully completed the projects within the deadlines.

Question: What position do you prefer on a team working on a project?

Ans: It doesn't matter till I learn something new in every project.

Question: What irritates you about co-worker?

Ans: I believe in team work, even if I find anything irritating, I try to avoid it unless it personally affect me.

Question: Tell me about your ability to work under pressure?

Ans: I keep my cool while working under pressure, Multi tasking and patience usually helps in this kind of situations.

Question: How do you see yourself five years from now?

Ans: I see myself in a senior position managing important portfolio of this company.

Question: what are your expectation from this job?

Ans: Professional advancement & good future.

Question: Do you have any question for us?

Ans: When can I join? 

MINUS/EXCEPT in Microsoft Access

Microsoft Access is not supporting Minus or Except.

If , we want to find the difference in data between two tables we can use the below syntax in Access.

--MINUS IN ACCESS

SELECT

*

FROM

Table1

WHERE

(((Exists (SELECT * FROM Table2 WHERE Table1.field1 = Table2.field1 AND Table1.field2 =Table1.field2………………))=False));

Example:

                                                                            

Suppose we have two table name SRC and TGT and we want to find difference of the records which are not in target.

In Oracle we can write the simple query and can find the difference.

--Minus in  SQL

SELECT EMPNO,ENAME FROM SRC

MINUS

SELECDT EMPNO,ENAME FROM TGT;

But, Since ACCESS is not supporting Minus/Except. So we can find the difference between SRC and TGT table Using Below SQL.

SELECT EMPNO,ENAME FROM

SRC

WHERE

(((Exists (SELECT EMPNO,ENAME FROM TGT WHERE SRC.EMPNO = TGT.EMPNO AND SRC.ENAME =TGT.ENAME))=False));

Populate SCD3 from SCD2.

SELECT * FROM
(
SELECT A.SK,A.EID,A.ST_DT,A.ED_DT  FROM EMP2 A
INNER JOIN
(SELECT MAX(ED_DT) AS N_ED_DT , EID FROM EMP2 GROUP BY EID) B
ON A.ST_DT=B.N_ED_DT

UNION ALL

SELECT A.SK,A.EID,A.ST_DT,A.ED_DT   FROM EMP2 A
INNER JOIN
(SELECT MAX(ED_DT) AS N_ED_DT , EID FROM EMP2 GROUP BY EID) B
ON A.ED_DT=B.N_ED_DT
) C
ORDER BY C.EID

Virtual Column in Where clause

Question: Can we call virtual column in the where clause.

Answer: Yes, we can.

Description:

Suppose we have the Standard EMP table (as per oracle Scott user) and want to assign the job title on the basis of SAL column if SAL between 1000 and 3001 then JOB_TITLE is 'JUNIOR' and if SAL between 3001 and 5000 then JOB_TITLE is 'SENIOR' else JOB_TITLE is 'TRAINEE'.

                                                                  Now, we want to fetch the records that have the JOB_TITLE as ‘TRAINEE’ only.  Here JOB_TITLE is the virtual column (this is not physically in EMP table), so we can’t filter directly in WHERE clause.

Ok let’s try,

SELECT EMPNO, ENAME, SAL, DEPTNO,

CASE

 WHEN SAL >=1000  AND SAL<=3000 THEN 'JUNIOR'

 WHEN SAL >=3001  AND SAL<=5000 THEN 'SENIOR'

 ELSE 'TRAINEE'

 END AS JOB_TITLE

FROM EMP

WHERE JOB_TITLE = 'TRAINEE'; 

The above query will return the below ERROR.

Error: Invalid column name 'JOB_TTILE'.

To resolve this Error we can write the same query in different manner.

Method 1

SELECT EMPNO, ENAME, SAL, DEPTNO,

CASE

 WHEN SAL >=1000  AND SAL<=3000 THEN 'JUNIOR'

 WHEN SAL >=3001  AND SAL<=5000 THEN 'SENIOR'

 ELSE 'TRAINEE'

 END AS JOB_TITLE

FROM EMP

WHERE

(CASE

 WHEN SAL >=1000  AND SAL<=3000 THEN 'JUNIOR'

 WHEN SAL >=3001  AND SAL<=5000 THEN 'SENIOR'

 ELSE 'TRAINEE'

 END AS JOB_TITLE) = 'TRAINEE'

Method 2

SELECT EMPNO, ENAME, SAL, DEPTNO, JOB_TITLE

FROM

(

SELECT EMPNO, ENAME, SAL, DEPTNO,

CASE

 WHEN SAL >=5000  AND SAL<=10000 THEN 'JUNIOR'

 WHEN SAL >=10001 AND SAL<=20000 THEN 'SENIOR'

 ELSE 'TRAINEE'

 END AS JOB_TITLE

FROM EMP

)

WHERE JOB_TITLE='TRAINEE'

In the Method 1 we try to take full virtual column in WHERE clause and in Method 2 we try to solve this problem with the IN LINE view.